EXTERNAL ANATOMY OF INSECTS. 715 



spiracles ; these in Epeira Diadema are three pair of small 

 black points : on the back of the abdomen also are four 

 pair, but in some species there are only two a : the most re- 

 markable, however, are exhibited by the cancriform spi- 

 ders before noticed b : in Epeira cancriformis, in the plate 

 which covers the abdomen, they are dark red spots with 

 an elevated rim and centre c exactly resembling spiracles, 

 except that they are not perforated ; there are twenty- 

 four of them, twenty arranged round the margin, and 

 four in a square in the disk. 



3. Organs of motion. In a former letter you were 

 told that several insects are enabled to leap by means 

 of organs in their abdomen ; I shall now describe such 

 of them as require further elucidation. I then said that 

 Podura and Sminthurns, two apterous genera, take their 

 leaps by means of an anal fo?-k d . In the former genus 

 the fork consists of a single piece attached to the under 

 side of the anus, and terminating in a pair of long slender 

 sharp processes which articulate with it and form the 

 fork or saltatorious instrument e . In Sminthurns the tines, 

 as they may be called, of the fork do not articulate with 

 the base, but are of the same piece and consist of two 

 joints, the terminal one being flat and obtuse f . Machilis 

 to the anal fork adds eight pair of ventral linear springs 

 (Elastes), which are covered with hair or scales, and ter- 

 minate in a bristle or two. I have on a former occasion 

 mentioned the natatorious laminae with which the anus 



* Treviranus. Arachnid. 23 — . b See above, p. 702, 706. 



c Plate XXIX. Fig. 26. represents one of them. 



«' Vol. II. p. 319—. 



e Plate XV. Fig. 14. M'. De Geer, vii. t. ii. /. 5, 10,21. 



{ Ibid. t. iii. /. 4, 14. 



